URBANA, Ill. — With all of the focus on Asian soybean rust during recent growing seasons, it’s understandable why soybean producers in the Midwest have invested time and money in preventing the spread of the yield-robbing disease.

But with rust yet to find its way as far north as Illinois’ farm fields, researchers at the University of Illinois (UOI) are urging producers to redouble their efforts in combating an older, more familiar foe: soybean cyst nematodes (SCN).

According to a recent survey conducted by researchers from the UOI’s Department of Crop Sciences, nearly 85 percent of all fields tested for the study revealed the presence of SCN, with half registering SCN levels that “exceeded the threshold for measurable economic damage.”

The damage caused by SCN is estimated at more than $250 million in yield losses annually, according to the study.

“The objective was to find out where the nematodes were present and what the population density levels were at those locations,” said Terry Niblack, professor of nematology at UOI, of the study that included testing 260 soybean fields across the state. “We also wanted to determine whether the populations of SCN could attack the resistant soybean varieties that are widely planted by growers.”

He said the Illinois fields averaged around 2,700 SCN eggs per cubic centimeter of soil, or five times the level considered enough to cause producers a financial loss. Five percent of the fields tested exceeded 10,000 eggs per cubic centimeter, which marks the point at which no soybean varieties can resist a SCN invasion.

“We also found that about 65 percent of the nematodes in our samples could attack the most common source of resistance found in varieties planted around the state. Almost 95 percent of the varieties available to farmers use that same source of resistance.

The conclusion that we came to was that simply growing any single resistant variety is not enough to control the problem,” Niblack said.

He went on to say that SCN populations in Illinois have adapted and changed in the last 15 years, pointing to a 1991 study that showed only about 30 percent of the sampled nematodes could attack that same source of resistance.

One major obstacle Niblack and other ag educators face is convincing producers of the need to be vigilant about the presence of SCN when plants often do not present visible symptoms of infestation. Most don’t move on the problem until yellow patches start encroaching on their plants, Niblack said.

“Many growers in Illinois think they do not have a problem, when they are actually suffering some significant yield losses,” he said. “If they are only getting 50 or 60 bushels per acre, they almost certainly should have some major concerns. With today’s high-yielding resistant varieties, farmers should be getting 75-80 acres per bushel.”

The study recommends that farmers with SCN problems rotate varieties with different sources of resistance, though that seems to pose a problem as all common varieties come from a source known as PI 88788.

“At the same time, we know that all varieties with the same source of resistance are not exactly alike,” Niblack said. “The reason is that there are a number of different genes involved in resistance. As a result, each variety has a different level of resistance to the nematodes. By rotating between different varieties using the same source of resistance, growers are still showing the nematodes something new.”

Niblack said that by rotating the highest-yielding varieties containing the highest levels of SCN resistance, profits could be increased by up to $100 per acre, depending on the current market.

Results of the UOI’s Varietal Information Program for Soybeans are available at www.vipsoybeans.org